Burning and glazing result from the accumulation of high temperature as the workpiece progresses through a machine. These defects are usually caused by the cutter head, but may also result from the pressure rolls or bars.
The heat is primarily generated by rubbing between two surfaces, as shown in the photo on the right. The heat accumulated sufficiently to char the wood when the chair part stalled while being fed through a moulder. Because dry wood and wood products are generally poor heat conductors, the heat accumulates to a temperature high enough to cause charring in a relatively short time.
Accumulation of heat at the work piece surface also causes glazing; it also results from high temperatures and pressures that occur during wood machining. Glazing generally results from the flow or smearing of the lignin and/or wood extractives over the freshly machined surface. The glazed surface may reflect light more than an unglazed surface. The greater light reflection results from the smooth layer of materials over the surface, which may reduce adhesives and finishing properties of the surface.
As with raised grain ( CabinetMaker, May 2006) and fuzzy grain ( CabinetMaker, September 2006), burning and glazing result from one or more contributing factors, such as tool geometry, machining parameters, moisture content and species. Each of the factors may contribute more or less to the defects or in combination with the others.
Machine setup includes a number of the aforementioned factors. The pressure rolls, feed rolls, and pressure bars and rolls need to be adjusted properly so the stock doesn't stall while being fed through the machine. The stock needs to be fed fast enough not to accumulate heat. Feed rate, revolutions per minute (rpm) and number of flutes or knives in the cutter head should be adjusted for 16-60 knife marks per inch for a finish machined surface.
Tool geometry is another major factor that may affect burning and glazing. Low rake and clearance angles cause high cutting forces which, in turn, cause higher frictional forces and additional heat. Wood is a resilient material. Low clearance angles, in particular, cause more compression of the machined surface. As the wood recovers, it rubs more on the clearance face of the tool, generating more heat.
The evidence of low clearance angles is frequently apparent along pattern cutting edges. Charred or darkened material can frequently be observed behind the cutting edge where the rake and/or clearance angles are too small. Dull tools generally accentuate burning or glazing from small rake and clearance angles, which tend to dull faster.
Species also a factor
Wood species also contribute to burning or glazing. Wood products with high amounts of extractives usually have more glazing. For instance, the softwoods with resin, hardwoods with gum, and tropical and domestic woods with high extractive contents all burn or glaze more than species low in these substance contents.
Other species, such as cottonwood, are more resilient than others and rub more on the sliding interfaces, particularly the clearance face, between the workpiece and tool. Burning or glazing can occur with any wood species if proper machine setup and tooling are not maintained.
Moisture content also affects the degree of burning, charring or glazing. Generally, green wood doesn't burn or glaze, but can if the right conditions occur such as the feed stalling for a long enough time for the heat to accumulate. The softening point of lignin increases as pressure and moisture content increase. Although dry wood may be a better heat insulator than higher moisture content or green wood, wood at a higher moisture content may char or glaze more because lignin softens at lower temperatures as pressure and moisture content increase. Cooling or drying causes the lignin and various wood extractives to firm up or harden in the new position, which is often a layer over the wood surface.
Reduced bonding properties
The literature also indicates that oxidation of the cellulose or wood occurs when conditions are sufficient to cause burning, charring or glazing. This oxidation is in addition to the firm up or hardened material. All of these conditions can reduce the bonding properties between the wood surface and adhesives or finishing materials. Because the machined wood has oxidized and firmed up at and near the surface, a subsequent machining process is usually required to correct the burning or glazing.
The burned or glazed material can be removed by knife cutting or wood sanding processes. However, care should be taken not to accumulate heat when removing a burned surface. Feed rates should be as fast as possible to produce a subsequent satisfactory surface when knife cutting or sanding.
A sequence of sanding grits from coarse to fine may be required to remove the hardened surface and then improve surface quality. Refrigerated and/or compressed air may also help to reduce burning, charring or glazing. Again, as with other wood machining defects, moderate tool geometry and machine settings generally produce a satisfactory surface when machining wood or wood composites.
Summing up, proper tool geometry, woodworking machine settings, tool and machine maintenance, and controlling workpiece moisture content are all paramount to minimize wood machining defects.
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